1. Field of the Invention
This invention relates generally to a method for employing an effectiveness approach to model the performance of a fuel cell stack humidification device and, more particularly, to a method for employing an effectiveness approach to model the performance of a fuel cell stack humidification device so as to determine the amount of water transferred by the humidification device to provide humidification device control and/or design.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode side catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode side catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes (catalyst layers) typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, the gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and also helps in uniform reactant and humidity distribution. MEAs are relatively expensive to manufacture and require certain humidification conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as reaction by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
A fuel cell stack includes a series of bipolar plates (separators) positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include anode side and cathode side flow distributors (flow fields) for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membranes within a fuel cell need to have certain water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The flow of reactants through the gas flow channels has a drying effect on the membranes, most noticeably at an inlet of the flow. However, the accumulation of water droplets within the gas flow channels could prevent reactants from flowing therethrough, and cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the gas diffusion media, is particularly troublesome at low stack output loads.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and perhaps liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the membrane is absorbed by the membrane and transferred to the cathode air stream at the other side of the membrane.
During fuel cell system operation, it is desirable to provide the cell membranes with a certain amount of water content so they are not too wet or too dry. A stack with membranes that are too wet at low current density could have instability issues. A membrane that is too wet may cause problems during low temperature environments where freezing of the water in the fuel cell stack could produce ice that blocks flow channels and affects the restart of the system. Membranes that are too dry may have too low of a protonic conductivity, resulting in lower stack voltage and performance. In addition, membranes that are too dry at the next system restart can affect restart performance and may reduce stack durability. Therefore, there is a need in the art to provide a method for maintaining the fuel cell membrane at a level of humidification which is neither too wet nor too dry. Further, it would be useful to develop a method that estimates the humidification level of the fuel cell membrane without using an expensive relative humidity (RH) sensor and that provides a convenient and ready visualization of the physical significance of the membrane humidifier performance, i.e., the performance of a WVT unit or similar device.